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THE LAKES OF IOWA,—PAST AND PRESENT. 149 
Meeting no such obstruction on a marshy side, the material 
thrust out accumulates just where the expansive force of the 
ice is spent. This process repeated year after year, from 
age to age, has cleared the bottom of the lakelets of their 
bowlders and other materials, and piled them up in circular 
ridges upon their shores; and these are the “walls” which 
have excited so much wonder. It has been observed that 
the embankments are heaviest on the sides opposite the pre- 
vailing winds. This may be accounted for, at least in part, 
by the fact that the ice being burdened with the material to 
which it has frozen fast, would thus be floated against those 
shores when the spring floods had raised the water of the 
lakes; and in part also by the farther fact that the dashing 
of the waves would be most constant against those shores. 
Thus it will be seen that whatever was originally upon the 
bottom, whether bowlders, gravel, sand, or mud, has been 
carried to the shore, and we find the embankments composed 
of all these materials arranged in perfectly natural disorder. 
If bowlders were numerous, the embankment is largely com- 
posed of them. If sand prevailed, a broadly rounded em- 
bankment is formed, just such as we should expect from 
such material; and where a peat marsh extends out into the 
land, an embankment of turf is thrown up at the water’s 
edge, which, being supported by living rootlets, is frequently 
high and very narrow. The latter are somewhat numerous, 
and are often called beaver-dams ; but this is also a miscon- 
ception, because beavers never attempt to dam still waters. 
ey dam running streams to obtain pouds of still water. 
Thus we see that the same natural force placed the bowlders 
im the embankments that brought them down from their 
northern homes, namely, the expansive power of ice. 
If its crust should remain perfectly stable long enough, the 
earth would become nearly a perfect sphere by the disinte- 
gration of its exposed substance, and the levelling force of 
Stavitation. It is true that its inequalities of, surface are 

